JC: Fire ecology of Rocallaura Fires (Lleida, Spain), June-July 2016

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Versión en castellano

Author: Núria Prat, PhD on Environmental Science at University College Dublin. International Projects Manager at Pau Costa Foundation.

The ‘Rocallaura 1’ wildfire took place on June 23rd close to the Rocallaura village (Spain) propagating through a pine forest (Pinus halepensis). Almost one month later, there was a second wildfire, ‘Rocallaura 2’, on July 19th. This second fire was ignited within the perimeter of the Rocallaura 1 fire. This time the fire spread through the pine forest but also through crop fields, olive plantations and finally getting close to the urban areas of Nalec and Rocafort de Vallbona. A total of 817 ha were consumed by those fires.

Both fires took place in mountain Mediterranean forests, in areas were no forestry-management actions were carried out for decades. From the tree scars we can tell that the area did not have a fire since the Spanish Civil War in the 40s. In the last years, winter and summer droughts have affected the area, and in general all the south-west region of Catalonia. As a consequence, before the fire, the fuels were highly available, enabling the spread of the fire during the events.

Walking on the affected area a month and a half after the second wildfire event, we could observe some short-term effects on the ecosystem. We then decided to write this post to share and discuss these effects with all of you. It is important to highlight that the following lines are only observations, which have not been scientifically tested in the area:

  • The fire spread at a variable intensity, causing a diversity of effects on the ecosystem and creating mosaic. We observed that the changes on the fire intensity are imprinted in the landscape. The diversity of fire intensities created a mosaic of burnt patches within the dense and homogeneous forest mass existing before the fires. The subsequent mosaic combines green, burnt areas and crops. The burnt areas are now new spaces for plants colonising from the contiguous unburnt forest. Overall, causing the regeneration of the forest and enhancing species biodiversity.
  • A few fire-adapted strategies from Pinus halepensis were visible in the areas burnt with high and low severity. The area affected by the leading front of the fire was consumed with the highest intensity. In this area trees were completely burnt leaving behind only dead tree trunks, ashes on the soil and open spaces where sunlight could reach the ground. We also observe that this area is now a “nursery” of pine seeds. During the fire, the cones were heated causing the release and spread of the seeds. Since then, the seeds are on the ground mixed with the soil and ashes and ready to germinate.
Forest area burnt with a high intensity. Photo: PCF
Forest area burnt with a high intensity. Photo: PCF

On the right and left flanks, the fire intensity was low. As a result, only a portion of the trees were burnt. In those areas, the fire propagated mainly consuming the understory vegetation. As a consequence, many tree crowns remained, impeding the penetration of the sunlight to the ground’s surface, which in this case is a mixture of soil, ashes and pine needles.

Forest area burnt with a low intensity. Photos: PCF
Forest area burnt with a low intensity. Photos: PCF
  • A layer of ash and char was accumulated on the top soil and it is now protected from erosion under a layer of pine needles. The ash and char bring minerals and nutrients to the soil fostering the accumulation of organic matter. These conditions are observed mainly under the trees burnt with low severity. The fire consumed most of the understory fuel.
Understory of an area burnt with low intensity. The ground is covered by a layer of pine needles. Photo: PCF
Understory of an area burnt with low intensity. The ground is covered by a layer of pine needles. Photo: PCF

Afterwards, the remaining char and ashes were accumulated on top of the soil providing minerals and nutrients for plants. The pine needles felt on the ground afterwards, protect the soil, char and ash particles from erosion processes. If no post-fire management measures are taken to remove the burnt logs, most of the partially consumed material will degrade forming soil organic matter.

Close-up of the ground a month and a half after the second fire. The ashes and carbon remain underneath the pine needles. Photo: PCF
Close-up of the ground a month and a half after the second fire. The ashes and carbon remain underneath the pine needles. Photo: PCF

 

  • The post-fire accumulation of carbon on the soil can potentially contribute to mitigate the effects of climate change. During the fuel combustion, part of the carbon is released to the atmosphere as greenhouse gasses, mainly as CO2 and CO. However, another part is transformed to organic and mineral carbon (char and ashes). Both compounds remain in the ecosystem and contribute to the carbon-sink function of the forest. What is more, some recalcitrant carbon compounds are formed during the combustion of wood and are considered very resistant to degradation (e.g. black carbon), that can remain in the system much longer than green vegetation itself.
  • Despite the lack of rain since the fire, some species start sprouting after a few weeks. Species such as holm oak (Quercus ilex) sprout a month and a half after the fire. This example shows the adaptation of Mediterranean species to forest fires. It should then be considered whether reforestation is necessary after a wildfire, as human intervention will potentially have a higher impact than the ecological fire adaptation developed by Mediterranean plants over millennia.
Example of a holm oak naturally sprouting in the area burnt with a high intensity of Rocallaura 1. Photo: PCF
Example of a holm oak naturally sprouting in the area burnt with a high intensity of Rocallaura 1. Photo: PCF
  • Some bird species take advantage of the fire effects on vegetation and landscape caused by the fire. Birds such as the great spotted woodpecker (Dendrocopos major) or the great tit (Cyanistes caeruleus) usually remain in the burnt areas if the burnt tree trunks are not removed. Since the presence of fungi and insects is usually higher in dead trees, these are areas of food source for insectivorous birds. We observed trunks without bark indicating bird activity in areas were fire intensity was high.

Birds can take advantage of the new mosaic: they can keep living in the green areas and explore the burnt ones. Consequently, protecting green trees from fungi and insect plagues.

The nests of some singular birds, such as the Northern goshawk (Accipiter gentilis) could had been affected by the fires. Nevertheless, since Northern goshawk hunts in open areas, the fire perturbation could cause an increase of the number of this bird in a mid and long term scale.

The open areas can also restore habitat for species such as the Ortan bunting (Emberiza hortulana), black wheatear (Oenanthe leucura) or the Bonelli’s eagle (Aquila fasciata), especially in areas were the dense forest replaced open spaces in the last decades. Finally, it should be mentioned that several studies indicate that the management of the burnt fuel (e.g. removal of dead tree trunks), could have a higher impact to bird populations than the fire perturbation itself.

Pine trees in the area burnt with a high intensity. The bark of some of the trees have been removed by birds looking for food sources. Photo: PCF
Pine trees in the area burnt with a high intensity. The bark of some of the trees have been removed by birds looking for food sources. Photo: PCF

During the next weeks we will publish a second post talking about the Rocallaura fires and the re-ignition of the fire that took place 26 days after the suppression of the first fire. We will discuss what occurred with the smouldering fire during that month.

We would like to thank Francesc Moncasí for his contribution on the section of fire effects on birds!

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Autor: Pau Costa Foundation

La Fundación de Ecología del Fuego y Gestión de Incendios Pau Costa Alcubierre (PCF) tiene la ambición de ser una plataforma de información en gestión de los incendios forestales, a la vez que un instrumento de difusión e investigación en la ecología del fuego.

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